1. Field of the Invention
The present invention relates to an improved process for the selective cleavage of cumene hydroperoxide (CHP) into phenol and acetone by the action of an acid.
2. Description of the Background
Phenol is mainly produced internationally from cumene. Cumene is first oxidized to cumene hydroperoxide (CHP). For reasons of selectivity, cumene conversion rates of 20-30% by weight are generally achieved. In a subsequent step, called concentration, the CHP content in the oxidized material is customarily concentrated to 65-90% by weight. What is termed technical-grade CHP is produced. In the subsequent cleavage reaction of the technical-grade CHP product, by action of an acid, usually sulfuric acid, phenol and acetone are produced. After neutralization of the cleavage product, the products are isolated from the cleavage product essentially by distillation in what is termed the work-up stage of a phenol plant.
Both the oxidation reaction and the cleavage reaction are accompanied by unwanted byproduct formation. The cleavage reaction is of particular importance for the selectivity and the yield of the overall process. Parallel to the cleavage of CHP into phenol and acetone, the dimethyl phenyl carbonyl (DMPC) previously formed in the oxidation is dehydrated to .alpha.-methylstyrene (AMS). AMS can be hydrogenated in the work-up stage of the process to cumene and thus can be recycled back to the oxidation stage. However, in the CHP cleavage reaction, AMS polymerization reactions or addition reactions between AMS and phenol occur, which reactions lead to the formation of high-boilers (polyAMS and cumylphenols), as a result of which the selectivity and yield of the overall process are substantially adversely affected. The cleavage reaction must, therefore, be conducted industrially in such a manner that the formation of the high-boilers is to the greatest possible extent suppressed.
The industrial cleavage procedure has been described a number of times. Generally, the cleavage reaction is conducted in an ideally mixed apparatus. The heat released in the highly exothermic CHP cleavage reaction is dissipated either by evaporating acetone (evaporative cooling, cf U.S. Pat. No. 5,463,136) or by external coolers such as described, for example, in U.S. Pat. No. 4,358,618. U.S. Pat. No. 4,358,618 describes how the selectivity of such a cleavage can be improved by downstream tubular reactors. The procedure is as follows. In an ideally mixed main reactor, technical-grade CHP is cleaved to residual CHP concentrations of 0.5-5% by weight by adding an acid at temperatures in the range from 50-90.degree. C. Under these conditions, at least 40% by weight of the DMPC present reacts with CHP to form dicumyl peroxide (DCP) and water. In the downstream first tubular reactor, the CHP is cleaved to residual contents below 0.4% by weight, and the temperatures in this reaction are similar to those in the main reactor. In the second tubular reactor, finally, the DCP formed previously is cleaved into AMS, phenol and acetone. Temperatures in the range of 120-150.degree. C. are established. Therefore, the main reactor and the first tubular reactor can be summarized as "CHP cleavage" and the second tubular reactor can be called "DCP cleavage". The idea of thermally post-treating the cleavage product after the actual CHP cleavage reaction, as described in U.S. Pat. No.4,358,618. in a further tubular reactor has long been known and has been described in U.S. Pat. No.2,757,209. Temperatures above 100.degree. C., preferably from 110-120.degree. C., are specified for the DCP cleavage. The purpose of this thermal post-treatment is then the complete dehydration of DMPC to form AMS. In addition to the high-boilers already formed in the CHP cleavage, further high-boilers are formed in the DCP cleavage.
Published patent application SU 11 31 865 A also describes a two-stage process, wherein, in the first stage, the cleavage of CHP is performed, while, in the second stage, DCP formed in the first stage is cleaved. SU 11 31 865 A teaches, furthermore, that for heat dissipation, based on each stage, recycling product from one stage to the feed of the other stage in a ratio of 20:1 and not feeding the sulfuric acid used as catalyst until the second stage, so that further recycling of cleavage product from the DCP cleavage reaction (second stage) to the feed of the CHP cleavage reaction (first stage) in a volumetric ratio of 1:1-:10 is necessary in order to transfer the acid to the first stage.
Studies conducted by the inventors have shown that the rate of cleavage of cumene hydroperoxide dCHP/dt is proportional to the CHP concentration. Thus, for a given conversion rate, the space-time yield in an ideally mixed reactor as described in U.S. Pat. No. 4,358,618 is always lower than in a tubular reactor.
The volume of such an ideally mixed apparatus is, therefore, always greater than that of a tubular reactor if the same CHP flow rates in each case are to be cleaved to give the same CHP final contents in each case. The amounts of cleavage product having residual CHP contents which are found in an ideally mixed apparatus are correspondingly greater, compared with a tubular reactor. However, for safety reasons, it is advantageous to keep the volume in cleavage reactors as low as possible and simultaneously provide a high heat transfer surface between reacting technical-grade CHP and cooling medium. This is achieved if the cleavage is conducted in reactors having plug flow characteristics, that is, for example, tube-bundle heat exchangers. In this case, the product can flow both on the tube side and in the shell space if, here, plug flow characteristics are established by special internals (deflection baffles). Small reaction volumes and large heat exchange surfaces, i.e. reactors having large specific surface areas per unit volume for heat transfer, ensure that even in the event of failure of, for example pumps, despite the highly exothermic cleavage reaction of CHP, no safety-critical states occur.
The use of a plurality of series-connected tube-bundle heat exchangers for the cleavage of technical-grade CHP has already been described in DBP 1 112 527. In this case, CHP is dispersed in excess sulfuric acid and the dispersion can then be passed through the coolers in which the heat of reaction is dissipated; sulfuric acid and organic phase are then separated from each other. The sulfuric acid is recycled and the cleavage product is neutralized and worked-up. By means of this so-called heterogeneous cleavage reaction, however, high selectivities cannot be achieved, since, in the circulating sulfuric acid, side reactions with the formation of high-boilers proceed to an increased extent.
Therefore, similarly to U.S. Pat. Nos. 4,358,618, 5,254,751 also describes a homogeneous CHP cleavage, that is to say the sulfuric acid used only in small amounts dissolves in the reaction mass. In contrast to U.S. Pat. No. 4,358,618, according to U.S. Pat. No. 5,254,751, however, the CHP cleavage reaction is conducted in three series-connected coolers at from 45-75 .degree. C., these obviously being tube-bundle heat exchangers, in which the reaction mass flows through the shell space under plug flow conditions. The reaction product, in this case, is partially recirculated to achieve high selectivities, so that some of the CHP cleavage product arising downstream of the last cooler is recycled and admixed with the CHP stream flowing into the first cooler, the mass flow ratio of recycled circulated stream to the inflowing CHP stream, what is termed the circulation ratio .lambda., is said to be in the range from 10-25. Furthermore, an embodiment of the process is that in the effluent from the reactor system, from 0.3-1.5% by weight residual contents of CHP must be present. The cleavage product is then treated by being heating to a temperature of 80-110.degree. C. under plug flow conditions in order to convert the DCP formed in CHP cleavage into phenol, acetone and AMS.
FIG. 1 is an outline diagram of this CHP and DCP cleavage reaction described in U.S. Pat. No.5,254,751. In reactor 1, the CHP cleavage occurs under plug flow conditions. Some of the cleavage product is branched off via line 3 and, after addition of acid as catalyst via line 6, mixed with the feed stream or technical-grade CHP via line 4, before this mixture enters reactor 1. The remainder of the cleavage product is fed via line 5 to the tubular reactor 2 for DCP cleavage. The circulation ratio .lambda. is given by the ratio of circulation rate in line 3 upstream of the CHP admixture to the CHP feed stream via line 4. The cleavage product circuit is generated via a pump 7 (circulation pump). The cleavage product which, for example, flows freely from the circuit, is pumped by means of a further pump 8 to the downstream DCP reactor 2.
Although the use of reactors having plug flow characteristics gives, in principle, as described above, the advantage of higher space-time yields and thus smaller reaction volumes, because of the high circulation rates required, the volume of these apparatuses increases again, and, furthermore, large piping and pumps are required to implement these high circulation rates. A need, therefore, continues to exist for a more efficient process of cleaving CHP which provides also for a reduction of capital equipment costs.